The present invention generally relates to noise reduction circuits, and more particularly to a noise reduction circuit for use in a video signal recording and/or reproducing apparatus such as a video tape recorder in order to eliminate noise components.
Video tape recorders using a so called color-under recording system are widely used. In such a video tape recorder, a luminance signal is frequency modulated and recorded on a magnetic tape together with a frequency converted carrier chrominance signal having a frequency range lower than that of the luminance signal in accordance with the frequency division multiplexing technique. Such a video tape recorder has a tendency that the signal-to-noise ratio (S/N ratio) deteriorates with increasing frequency due to the increase of noise components having a so called triangular spectrum. In order to avoid this problem, the video tape recorder uses a noise reduction circuit which employs a preemphasis to the luminance signal to be recorded prior to the frequency modulation. In other words, a high frequency component of the luminance signal is extracted and added to the original luminance signal after suitable processing. As a result of the preemphasis, the high frequency component of the luminance signal is enhanced. At the time of reproduction, the same noise reduction circuit is used to perform a deemphasis, which is complementary to the preemphasis, on the demodulated luminance signal in order to complement the preemphasis. In the description hereinafter, the term emphasis will be used to include both preemphasis at the time of recording and deemphasis at the time of reproduction.
In recent years, there has been a need to increase the degree of emphasis mainly as a result of the broadened frequency range of the video tape recorder, for example from the conventional 3 MHz range to 5 MHz range. Generally, the increase in the degree of emphasis certainly increases the effect of noise reduction. However, if the degree of emphasis applied is excessive, the carrier frequency of the frequency modulated luminance signal would deviate too much. This would result in a so called inverted white peak in the reproduced picture. In other words, when the degree of preemphasis of the high frequency component of the luminance signal becomes excessive as a result of the preemphasis, the side band components of the frequency modulated luminance signal interfere with the frequency converted carrier chrominance signal to such an extent that the reproduced color signal is deteriorated. In order to prevent the excessive frequency modulation of the luminance signal, a white clip circuit is usually used to eliminate from the frequency modulated luminance signal the frequency component deviating to such an extent that it interferes with the frequency converted carrier chrominance signal. However, the use of such a white clip circuit is associated with a possibility of causing a poor picture quality due to the fact that the circuit may cut too much of deviated frequency component.
In order to avoid this problem, the conventional noise reduction circuit uses a limiter circuit which limits the amplitude of the high frequency component of the luminance signal at a predetermined level. Such a limiter circuit generally has an input versus output characteristic which changes the slope of the characteristic curve stepwise in two stages as the amplitude of the input signal increases.
Conventionally, there is a noise reduction circuit as disclosed in the Laid-open Japanese Patent Application No. 27178/1988 in which the applicant is the assignee of the present application. At the time of recording, the noise reduction circuit operates as a preemphasis circuit and an input luminance signal is subjected to amplitude limitation in a limiter circuit after passing through a high pass filter circuit. An output signal of the limiter circuit is on one hand looped back to an input port of the high pass filter via a feedback loop including a multiplier for modifying the amplitude of the signal passing therethrough by a predetermined coefficient. The output signal of the limiter circuit is on the other hand is passed through a second multiplier for modifying the amplitude of the signal passing therethrough by another predetermined coefficient which defines the degree of emphasis and an output luminance signal applied with preemphasis is obtained by summation of an output signal from the second multiplier and the original luminance signal. At the time of reproduction, the output signal thus obtained from the second multiplier is subtracted from the input luminance signal which is already applied with preemphasis and an output signal compensated with the effect of preemphasis is obtained.
In such a conventional noise reduction circuit, it is known according to the Laid-open Japanese Patent Application No. 27178/1983 that both the random noise and a horizontally streaking noise which appears responsive to an input of an impulsive noise can be effectively suppressed by setting the time constant of the high pass filter as EQU T&gt;Ts&gt;T/(X+1)
where T stands for a time constant corresponding to a lower limit frequency at which the effect of preemphasis or deemphasis becomes effective.
As will be explained in detail later with reference to the drawings, such a noise reduction circuit responds to an incoming luminance signal with a time constant Ts if the level of the high frequency signal component in the luminance signal exceeds a predetermined level set by the limiter circuit, while the circuit responds with a time constant T/(X+1) when the level of the high frequency component signal in the incoming signal is decreased below the level set by the limiter circuit. As the time constant Ts is set larger than the time constant T/(X+1), the noise reduction circuit at first responds with the large time constant in correspondence to the portion of the high frequency luminance signal component exceeding the level of the limiter circuit and then responds with the small time constant in correspondence to the portion of the high frequency signal component which is smaller than the level of the limiter circuit. Thus, the time period in which the noise reduction circuit operates with the time constant Ts is usually longer than the time period in which the noise reduction circuit operates with the time constant T/(X+1) . This results in an increase in the degree of emphasis in the relatively lower frequency range of the luminance signal. However, such an operation is contradictory to the purpose of the emphasis to enhance or diminish only the high frequency signal component of the luminance signal.